1. Technical Field of the Invention
The present invention relates to a regenerative control apparatus for vehicles equipped with a lock-up clutch for complete engagement with a hydraulic torque converter.
2. Prior Art
Regenerative control apparatuses for use in vehicles equipped with a lock-up clutch have been suggested.
Japanese Unexamined Patent Application Publication No. 07-310566, for example, suggests starting regenerative braking after bringing a lock-up clutch into a state of complete engagement, and stopping the regenerative braking after bringing the lock-up clutch into a state of complete disengagement when an engine speed becomes lower than a fuel-cut releasing speed.
Further, Japanese Unexamined Patent Application Publication No. 2004-224110 suggests a hybrid electric vehicle which performs regenerative braking after bringing a lock-up clutch into an engagement state including a complete engagement state or slipping state.
Further, Japanese Unexamined Patent Application Publication No. 2000-134713 suggests a hybrid electric vehicle which starts regenerative braking after bringing a lock-up clutch into a complete disengagement state or slipping state.
Specifically, in any of the regenerative control apparatuses for vehicles equipped with a lock-up clutch suggested in the patent literatures provided above, the state of the lock-up clutch is established first, and then according to the established state, execution or stoppage of regenerative generation is determined.
However, the regenerative braking control as disclosed in Japanese Unexamined Patent Application Publication No. 07-310566, and the mode for performing regenerative braking after bringing a lock-up clutch into a complete engagement state as disclosed in Japanese Unexamined Patent Application Publication No. 2004-224110, have arisen such disadvantages as provided below. In particular, in both of the cases, although good regenerative braking may be performed in a high-speed range of a vehicle, if some causes for drastically increasing engine load torque are given (e.g., a case where a hard stop is made in the complete engagement state of a lock-up clutch to lock the wheels), the engine speed may drastically decrease. Accordingly, the engine-generated torque may be decreased, possibly falling into a state of engine stall.
On the other hand, the regenerative braking control as disclosed in Japanese Unexamined Patent Application Publication No. 2000-134713, and the mode for performing regenerative braking after bringing a lock-up clutch into a slipping state as disclosed in Japanese Unexamined Patent Application Publication No. 2004-224110, have arisen such disadvantages as provided below. In particular, in these cases, although the engine stall described above is unlikely to be caused, there may be a problematic large energy loss in an automatic transmission and thus sufficient regeneration may not be expected. Further, as a result of drastic decrease of the engine speed due to regenerative braking, which necessitates stoppage of fuel cutting to prevent engine stall, fuel consumption may tend to be deteriorated.
In the regenerative control apparatuses for vehicles equipped with a lock-up clutch suggested in the patent literatures provided above, control for establishing some state in a lock-up clutch is executed in performing regenerative braking. In such a regenerative control, regenerative energy may be increased if regenerative braking is determined to be performed in a state where the coupling of a lock-up clutch has been strengthened. In return, however, the risk of engine stall may be increased. If, on the contrary, regenerative control is performed in a state where coupling of a lock-up clutch has been weakened, the risk of engine stall may be decreased. In this case, however, because the automatic transmission (typically, its hydraulic torque converter) is not being locked up, regenerative energy may not be effectively recovered. As a result, when regenerative generation is forcedly performed, the engine speed may tend to be drastically decreased to cause engine stall.